1. Field of the Invention
The present invention relates to a rubber composition for use in tires that improves cornering performance on snowy and icy roads and braking performance in the to-and-fro direction with reduced manufacturing cost, and more specifically relates to a pneumatic tire employing such a rubber composition.
2. Description of the Background Art
Use of studded tires is legally prohibited for the purpose of preventing dust pollution caused by the studded tires. Then, in place of the studded tires, studless tires have recently been used in cold climates. Technical development in the last few years achieves the braking performance of studless tires that is close to the braking performance of studded tires. One of the factors affecting the braking performance of studless tires is frictional force between the tread rubber of a tire and a road surface. The frictional force includes adhesive friction, digging friction and hysteresis friction.
It has been tried to enhance the adhesive friction of a rubber composition for tires in order to improve the braking performance of the tires. For example, the rubber composition is made softer to increase the area of contact between the tire and a road surface. In order to soften the rubber composition, the amount of a filler to be added to the rubber is decreased, any rubber component which is less prone to become hard at low temperatures, polybutadiene rubber or polyisoprene rubber for example is used, or a softener is further added. However, tires employing such a softener rubber composition deteriorate in terms of steering stability and abrasion resistance. Therefore, the rubber composition cannot be made too soft.
It has alternatively been proposed to replace a part of carbon black in a rubber composition with silica and silane coupling agent to decrease the modulus at low temperatures and thus enhance the adhesive frictional force. This method dramatically improves wet grip performance, ice grip performance and cornering performance in the lateral direction. However, the braking performance in the to-and-fro direction is insufficient.
In recent years, there has been fierce competition between tire manufactures to reduce the cost. With the demand for reduction in manufacture cost of tires and rubber composition for tires, a demand for recycle has been growing.
For example, it is known to blend, in order to reduce the cost, a tire rubber composition for use in manufacture of tires with an organic or inorganic fiber which is cut into short pieces. Blending of the organic or inorganic fiber can improve the elastic modulus and tear strength of the rubber composition and further decrease the amount of rubber used in a tire product. Accordingly, resource saving and weight saving are effectively achieved. However, the fiber material used in the rubber composition is usually produced by fiber manufactures by cutting a fiber which is newly spun for the purpose of short fiber reinforcement. Then, the manufactured rubber composition is likely to become costly.
It is also known to blend a tire rubber composition with an inorganic reinforcing agent such as calcium carbonate, mica, clay and the like. These inorganic reinforcing agents are relatively cheap so that the manufacture cost of tires can be reduced. However, depending on the amount of the blended inorganic reinforcing agent, the resultant tires could be deteriorated in performance. Moreover, depending on the blended amount, the specific gravity becomes greater and reduction of cost per volume might not be expected.
In addition, in terms of recycling of raw materials, reclaimed rubbers are under study. However, the studies on reclaimed rubbers have not achieved an effective reduction in the cost of manufacturing tires. Moreover, a technique for blending resin or the like with used paper is disclosed as a method of recycling the used paper. For example, Japanese Patent Publication No. 57-43575 discloses a composite produced by blending thermoplastic resin and synthetic rubber or natural rubber with stearic acid as a lubricant, fusing the blended materials, and kneading chopped pieces of used paper with the fused product in the liquid phase.
Japanese Patent Laying-Open No. 11-217466 proposes a method of manufacturing a rubber reinforced by used paper, the reinforced rubber manufactured by melting the used paper in an alkaline solution in advance, blending the used paper with a rubber latex, and thereafter coagulating the rubber.
These techniques are complex regarding the process of manufacturing a rubber composition and have not achieved an effective reduction of manufacture cost. In particular, it has not been possible to obtain a rubber composition for use in tires that improves cornering performance on snowy and icy roads and braking performance in the forward and backward directions with manufacture cost reduced.